1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for steerable beam antennas, and more particularly to periodic resonance structures that can be used for steering antenna beams.
2. Description of the Related Art
Periodic resonance structures may be found in a wide variety of RF applications. One example of a periodic resonance structure is a frequency selective surface (FSS). An FSS is conventionally designed to either block or pass electromagnetic waves at a selected frequency. These types of surfaces are essentially periodic resonance structures that are comprised of a conducting sheet periodically perforated with closely spaced apertures, or may be comprised of an array of periodic metallic patches. FSS structures can generally be separated into two broad categories, namely inductive and capacitive type geometries. An inductive FSS operates in a manner similar to a band-pass filter. A capacitive FSS, behaves in a manner that is similar to a band-stop filter. When the periodic elements comprising an inductive FSS are at resonance, the FSS will pass RF signals that are at or near the resonant frequency. In contrast, the capacitive FSS will reflect signals at or near the resonant frequency of the elements.
A typical capacitive FSS is constructed out of periodic rectangular metal patches disposed on a planar substrate. By comparison, an inductive type FSS is typically constructed using periodic rectangular apertures which are formed by perforating a metal sheet that has been deposited on a substrate. Many other types of FSS element configurations are known, including circles, Jerusalem crosses, concentric rings, mesh-patch arrays or double squares supported by a dielectric substrate. Depending upon the geometry selected, these can combine features of inductive and capacitive elements and can beg used to provide desirable frequency responses. U.S. Pat. No. 3,231,892 describes some basic FSS geometries and one potential application for an FSS type periodic resonance structure. Notably, signals that are blocked by a FSS are typically reflected away from the FSS, but the reflected direction is often not a matter of concern for the designer.
Another type of periodic resonance structure is a reflectarray. A reflectarray is typically comprised of an array of resonantly-dimensioned microstrip antenna radiator patches that are closely spaced above a ground plane. Conventional electronic phase shifters can be provided for shifting the phase of an incident RF signal received by each antenna radiator patch and then retransmitting the signal, usually via the same antenna radiator patch. For example, diode switches can be used to control a transmission line structure to vary a phase shift. The phase shifts of the individual resonators create a phased array effect that can be controlled to determine the direction of a redirected beam of RF energy. One example of a reflectarray is disclosed in U.S. Pat. No. 4,684,952 to Munson et al. However, alternative arrangements are also known in the art.